How much louder is the intensity of sound at a rock concert in comparison with the sound of a whisper, if the two intensity levels (in dB) are 120 and 20, respectively?

What my professor said was that the rock concert was 100 dB greater than the whisper because

120dB-20dB = 100dB and thus 1010 times louder, but I can't get the math to work out properly.

Perhaps you forgot the superscript, but I believe you instructor probably said it's 1010 times louder, not 1010 times louder. (1010 would be correct.)

I did

100dB = 10log(I/10^-12)

10 = log(I/10^-12)

10^10 = I/10^-12

I = 0.01

Your calculations might correspond to the intensity that is 100 dB louder than the human threshold of hearing (depending on which units you are using for your 10-12 figure). But your instructor claimed that the concert was 100 dB louder than a whisper.